A novel mechanism of plant defense induction by Trichoderma hamatum via cell wall-derived elicitors from Sclerotinia sclerotiorum

Abstract

Aims Trichoderma is a filamentous fungus beneficial to crops and widely used as biofungicide. Its mechanisms of action as a biological control agent against phytopathogenic fungi include mycoparasitism and the induction of plant defense responses. On the other hand, Sclerotinia sclerotiorum is an important pathogen for Brassica crops, effectively suppressed by Trichoderma. The aim of this work is to determine whether T. hamatum can release elicitors from the cell wall of S. sclerotiorum that activate plant systemic defenses against the pathogen.

Methods Liquid fermentation of T. hamatum on S. sclerotiorum mycelium was performed. In the resulting fungal filtrates, chitinase and β-endoglucanase activities were quantified, along with the amounts of glucosamine and glucan oligomers produced. These filtrates were subsequently applied to the roots of broccoli plants (Brassica oleracea var. italica), which were later foliar-infected with the pathogen. Lesions produced were measured and different systemic defensive responses were evaluated through hormonomics, glucosinolate profiling and non-targeted metabolomics. Results In fungal filtrates of T. hamatum cultured on S. sclerotiorum, chitinase (7.56–8.32 units/mL) and β-endoglucanase (3.45 units/mL) activity was determined. These filtrates also contained the highest amounts of glucosamine (0.75 g/L) and glucan oligomers (43.8 g/L). When applied to broccoli plants, the filtrates triggered a systemic defense response that was effective against the pathogen. This response was mediated by the hormones jasmonic acid, isopentenyladenine and ethylene, leading to the accumulation of antifungal compounds in the leaves, including glucobrassicin, niacin and several fatty acids. This defensive induction was not observed with glucosamine oligomers. Conclusions Therefore, T. hamatum releases glucan oligomers from the cell wall of S. sclerotionum which may act as potential elicitors of systemic plant defenses.